The present invention relates to the chemical modification of single chain polypeptides by means of covalent attachment of strands of poly(ethylene glycol) PEG and similar poly(alkylene oxides) to single chain polypeptide binding molecules that have the three dimensional folding and, thus, the binding ability and specificity, of the variable region of an antibody. Such preparations of modified single chain polypeptide binding molecules have reduced immugenicity and antigenicity as well as having a longer halflife in the bloodstream as compared to the parent polypeptide. These beneficial properties of the modified single chain polypeptide binding molecules make them very useful in a variety of therapeutic applications. The invention also relates to multivalent antigen-binding molecules capable of PEGylation. Compositions of, genetic constructions for, methods of use, and methods for producing PEGylated antigen-binding proteins are disclosed.